Countersunk head screw

ABSTRACT

The invention relates to a countersunk head screw with an even number of ribs with countersinking blades on the underside of the screw head and projecting therefrom. Every other rib has a step at its end towards the screw shaft formed by shortening the rib concerned in relation to each adjacent rib.

BACKGROUND OF THE INVENTION

The invention relates to a countersunk head screw having ribs which arearranged on the conical underside of its screw head, project relative tothe underside, are present in an even number and have countersinkingcutting edges.

Such a countersunk head screw is able by means of its ribs to cut freethe countersink in a workpiece for the countersunk head of the screw.German Utility Model 92 02 650 and German Offenlegungsschrift 33 34 212,for example, disclose such countersunk head screws. A common feature ofboth countersunk head screws is that they have an even number of radialribs identical to one another, the important factor according to theformer publication being to provide a larger number of ribs, namely morethan 8. This is intended to permit good distribution of the screwingforces over the countersinking cutting edges formed by the ribs. In thelatter publication, it is a matter of configuring the ribs like amilling cutter. In both countersunk screws, all the ribs extend from theouter rim of the screw head to the screw shank.

Such countersunk screws are a mass product which is manufactured withtools which must have a long tool life. In this case, the cold formingof the underside of the screw head is effected by an embossingoperation. The configuration of the underside of the screw head has toallow for this, since the embossing tool forms the negative image of theunderside of the screw head and accordingly has to be designed withprominences and recesses, which then cause the material of the screwhead to flow during the cold forming and in the process are subjected tohigh loads.

SUMMARY OF THE INVENTION

The object of the invention is to configure the countersunk head screwwith regard to the conical underside of its screw head in such a waythat, while achieving a good cutting action on the workpiece to befastened and a favorable chip flow, the ribs have a configuration whichpermit a robust, rigid design for the embossing tool. According to theinvention, this is achieved in that the ribs are present in an evennumber, of which every second rib, at its end facing the screw shank,has a step which is formed by shortening the relevant rib relative tothe adjacent ribs.

On account of the shortening in each case of the second rib, a ribconfiguration is obtained in which a clearance space results between twounshortened ribs, which clearance space is formed by the latter and thestep of the shortened rib and is relatively large in the area where therib ends facing the screw shank run toward one another so that the endsof the ribs would come very close to one another there if every secondrib were not shortened. A correspondingly large projection of theembossing tool can project into this relatively large clearance space;that is, the embossing tool has no especially narrow projections whichtend to break away under prolonged loading. However, the ribconfiguration according to the invention not only makes it easier todesign the embossing tool in such a way as to ensure a long tool life,it also enables chips to collect in the clearance spaces when screwingthe countersunk head screw into a workpiece, which chips would otherwisehave to escape or could exert an undesirable pressure on the material ofthe workpiece on account of their volume.

The length of the relevant rib following the step is expedientlyselected in such a way that this length corresponds to about 2/3 of thelength of the adjacent ribs. At this length, the shortened rib, in itsouter area, is in particular able to participate fully in the cutting ofthe material of the workpiece when the countersunk head screw is beingscrewed in and can remove the material cut in this area together withthe chips arriving from the inside, while in the radially inner area thematerial cut there can largely remain in the abovementioned clearancespaces so that a substantially smaller volume of chips is to be conveyedaway in this area.

The countersunk head screw dealt with here is often used to screw aplate forming the workpiece to any support. If the plate in this case isthicker than the screw head of the countersunk head screw, it isexpedient to lengthen the unshortened ribs right into the area of thescrew shank until they extend right into the end of the screw shank;that is, they also provide a cutting action on the material in the areaof the screw shank for widening the through-hole for the screw. If, onthe other hand, the plate is only as thick as or is thinner than thescrew head, it is sufficient for the unshortened ribs, with their endturned away from the screw head, to merge into the conical underside ofthe latter or to end in the transition between screw shank and conicalunderside. In this case, the screw head performs a cutting action in theworkpiece, which cutting action involves the entire screw head, in whichcase the screw-shank end attached to the screw head then projectsdirectly from the plate and merges into the support for the plate.

There are various possibilities for the extension of the ribs. First ofall, the ribs may be made to run radially. A rectilinearly radial courseof the ribs facilitates the manufacture of the forming tool for the coldforming. However, it is also possible, while retaining the linearity ofthe ribs, to arrange the latter in each case in an offset manneressentially parallel to their center line. If this is done in thescrew-in direction, the conveying action of the ribs on the chips canthus be improved.

A similar effect is achieved when the ribs form a concave cutting edgein the screw-in direction. Apart from a rounded construction, the ribsmay also be configured for this purpose from two rectilinear partsadjoining one another via a bend.

In particular if the relevant workpiece is made of a friable material,it may be useful to deliberately not remove the chips but to press theminto the material of the workpiece in order to compress the material inthe area of the screw head. To this end, the ribs are given a concaveshape in the screw-in direction of the screw. If the abovementioned ribsoffset in parallel are used, these ribs may be offset in the oppositedirection to the screw-in direction so that the chips are conveyedradially to the inside by the ribs.

Depending on whether it is desired to cut more or less material belowthe screw head, the outer surfaces of the ribs may be configuredaccordingly. These outer surfaces may be made to run rectilinearly, inwhich case less material is cut than if the outer surfaces of the ribsare made to run in an arched manner. In this case, an increased cuttingaction results.

If it is desired to sink the screw head deeper relative to the relevantouter surface of the workpiece, the screw head may be provided with anessentially cylindrical rim which continues over the relevant ends ofthe ribs. In this case, a cylindrical runout of the conical countersinkis cut by the ends of the ribs, into which runout the screw head cansettle and thus if need be lies deeper with its outer surface than thesurface of the corresponding workpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention are shown in the figures, inwhich:

FIG. 1 shows a countersunk head screw in side view,

FIG. 2 shows the same countersunk head screw in a section along line A-Bin FIG. 1,

FIG. 3 shows a side view of a countersunk head screw having rib endsmerging into the conical underside of the screw head,

FIG. 4 shows a side view of a countersunk head screw having ribsextending right into the end of the screw shank,

FIG. 5 shows a side view of the countersunk head screw having ribslengthened parallel to the screw shank,

FIG. 6 shows a section similar to that according to FIG. 2 having offsetribs,

FIG. 7 shows a section similar to that according to FIG. 2 having curvedribs,

FIG. 8 shows a section similar to that according to FIG. 2 having bentribs.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows the side view of a countersunk head screw with the screwhead 1, adjoining which is the screw shank 2 with the thread 3. Thescrew head 1 has a conical underside having the cone 4 adjoining thescrew shank 2 and the following cone 5, the latter merging into thecylindrical rim 6 on the top side 7 of the screw head. Attached to theunderside of the screw head 1 are a plurality of ribs 8 and 9,specifically three longer ribs 8 and three shorter ribs 9, as apparentfrom FIG. 2 explained further below. The ribs 8 and 9 are embossed fromthe material of the screw head 1 by cold forming. The ribs 8 extend fromthe rim 6 to the edge 10, at which the cone 4 adjoins the screw shank 2.

On their side facing the screw shank 2, the shorter ribs 9 end in thestep 11, which defines the length of the ribs 9. On account of thepresence of this step 11, a clearance space 12 (also see FIG. 2) isobtained between the two adjacent ribs 8, which clearance space 12 has afavorable effect on the configuration of the forming tool embossing thescrew head 1, since this forming tool forms the negative image of theunderside of the screw head 1. In this negative image, projections areto be provided on the forming tool in order to shape the intermediatespaces between the ribs 8 and 9, of which in each case the projectionsof the forming tool which are present laterally next to a shorter rib 9are connected via a bridge, filling the clearance space 12, in theforming tool. The forming tool is thereby given high stability and thusalso a correspondingly long tool life. The length of the shorter ribs 9corresponds to about 2/3 of the length of the adjacent longer ribs 8. Itmay also be pointed out that, as FIG. 1 shows, the screw head 1 isprovided with the slot 13 for a screwdriver with blade. Instead of theslot 13, any other tool receptacle, e.g. a cross recess, may of coursealso be used.

FIG. 2 shows a section along line A-B in FIG. 1; it therefore shows theunderside of the screw head 1 having the symmetrically arranged shorterribs 9 and the longer ribs 8, which in each case are arranged offsetfrom one another by the same angle. In this case, the longer ribs 8extend from the rim 6 to the edge 10, at which the cone 4 merges intothe screw shank 2. On their side facing the screw shank 2, the shorterribs 9 end in the steps 11, which here, as also shown by FIG. 1, areslightly bevelled in order to facilitate the flow of the material of thescrew head 1 during the cold forming.

When the countersunk head screw shown in FIGS. 1 and 2 is being screwedin according to the arrow direction shown in FIG. 2, the frontcountersinking cutting edges 14 of the ribs 8 and 9 cut material out ofthe workpiece, as otherwise necessary by means of a countersink bit. Thescrew head 1 therefore assumes the role of a countersink bit. The chipscut out of the material of the relevant workpiece by the countersinkingcutting edges 14 can escape in the radial direction next to the ribs 8and 9; however, they can also collect in the clearance spaces 12 infront of the steps 11, in the course of which they utilize the clearancespace 12 present here.

FIG. 3 shows the side view of a countersunk head screw having a screwhead 1 whose unshortened ribs 15 merge at their end facing the screwshank 2 into the underside of the screw head 1, which is formed here bythe single cone 17. This is therefore an embodiment in which, with thescrewing-in of the countersunk head screw, the countersinking cuttingedges 18 of the screw head only come into contact with the relevantworkpiece when the cone 17 has been pressed a short distance into thematerial of the relevant workpiece. This is of advantage when therelevant workpiece into which the screw head is to be sunk is made of arelatively thin plate.

A further embodiment of a countersunk head screw is shown in side viewin FIG. 4, in which the outer surfaces with their countersinking cuttingedges 21 of the ribs 19 and 20 run in an arched manner to the outside.In this way, slightly more material is cut out of the relevant workpiecethan is actually necessary for a countersink, a factor which is ofadvantage when the screw head 1 is to be sunk slightly right below thesurface of the relevant workpiece. Here, the unshortened ribs 20 runfrom the rim 6 right into the area of the screw shank 2, their relevantend 22 running a short distance over the screw shank 2.

A modification of the configuration according to FIG. 4 is shown in FIG.5, in which the end 23 overlapping the screw shank 2 runs parallel tothe screw shank 2; that is, a cylindrical bore is cut out of thematerial of the relevant workpiece in the area of the end 23, thediameter of which bore corresponds approximately to the diameter of thethread 3 so that a through-hole for the thread 3 is thus obtained in therelevant workpiece. Here, the cylindrical rim 6 of the screw head 1 isat the same time the boundary 24 for the relevant ends of the ribs 8 and9. These ends lie with their boundaries 24 in a cylindrical envelopewhich conforms to the cylinder forming the rim 6. On account of thisconfiguration of the ribs 8 and 9, a corresponding cylindrical recess iscut into a workpiece when the screw head is being screwed in, into whichrecess the screw head settles, whereby its top side 7 is correspondinglysunk relative to the surface of the relevant workpiece.

A screw head 1 similar to the representation according to FIG. 2 isshown in FIG. 6, in which unshortened ribs 25 and shortened ribs 26 ineach case run offset essentially parallel to a vertical center line 29.On account of this arrangement of the ribs 25 and 26, the chips flowingbetween the ribs tend to be conveyed to the outside when the screw head1 is screwed into a workpiece in the arrow direction shown.

A similar configuration is shown in FIG. 7, in which the ribs 27 and 28run convexely with their counter-sinking cutting edges 30, taking thescrew-in direction indicated by the arrow direction as a basis, and thustend to direct the chips lying between the ribs 27 and 28 away to theoutside.

A modification of the configuration according to FIG. 7 is shown in FIG.8, in which the ribs 27 and 28 are provided with a bend, which is ofadvantage for constructing the relevant forming tool.

It may also be pointed out that the ribs 27 and 28 according to FIGS. 7and 8 may be of concave configuration, resulting in the effect that thechip material cut out by them is compressed to the inside, that is, itis not conveyed to the outside, a factor which may be of advantage forvarious applications, in particular if the material of the workpiece isfriable.

I claim:
 1. A countersunk head screw having ribs which are arranged onthe conical underside of its screw head, project relative to theunderside, are present in an even number and have countersinking cuttingedges, wherein every second rib, at its end facing the screw shank, hasa step which is formed by shortening said every second rib relative tothe adjacent ribs.
 2. The countersunk head screw as claimed in claim 1,wherein a length of said every second rib corresponds to about 2/3 ofthe length of the adjacent ribs.
 3. The countersunk head screw asclaimed in claim 1 or 2, wherein the unshortened ribs are lengthenedright into the area of the screw shank.
 4. The countersunk head screw asclaimed in claim 3, wherein the unshortened ribs, with their end turnedaway from the screw head, merge into the conical underside of thelatter.
 5. The countersunk head screw as claimed in claim 3, wherein theunshortened ribs, with their end turned away from the screw head, end inthe transition between screw shank and conical underside.
 6. Thecountersunk head screw as claimed in claim 3, wherein the ribs extendright into the end of the screw shank.
 7. The countersunk head screw asclaimed in claim 6, wherein the ribs, in their area of extension at theend of the screw shank, run parallel to the screw shank.
 8. Thecountersunk head screw as claimed in claim 1, wherein the ribs runradially.
 9. The countersunk head screw as claimed in claim 1, whereinthe ribs in each case run in an offset manner essentially parallel to avertical center line.
 10. The countersunk head screw as claimed in claim1, wherein the ribs form a concave or convex countersinking cutting edgein the screw-in direction of the screw.
 11. The countersunk head screwas claimed in claim 10, wherein the ribs consist of two rectilinearparts adjoining via a bend.
 12. The countersunk head screw as claimed inclaim 1, wherein the outer surfaces of the ribs run rectilinearly. 13.The countersunk head screw as claimed in claim 1, wherein the outersurfaces of the ribs run in an arched manner to the outside.
 14. Thecountersunk head screw as claimed in claim 1, wherein the screw head hasan essentially cylindrical rim which continues over the relevant ends ofthe ribs.